1. Field of the Invention
This invention relates to plasma ion implantation, and more particularly to a high voltage implantation method and apparatus in which the plasma is generated with the same pulse-power system that applies a high voltage to the target being implanted.
2. Description of the Related Art
Plasma ion implantation is a process in which irregularly shaped target objects are immersed in a plasma and biased negatively to attract positive ions from the plasma, which impact the targets with sufficient energy to be implanted into them. The intent is to improve the surface properties of the object in areas such as better wear characteristics and increased hardness. Numerous different applications for ion implantation are known; its advantageous uses include the rapid processing of ferrous materials, for the production of industrial tool dies with good wear qualities but at relatively low cost and mass per die.
A high voltage, typically greater than 50 kV, must be applied to the target to provide sufficient acceleration to the plasma ions to achieve a significant implantation depth of 0.1 microns or greater. Previous implantation systems have used independent mechanisms to produce a continuous plasma, and to bias the target. One patent in this area, U.S. Pat. No. 4,764,394 to Conrad, uses a discharge between a cathode inserted into a plasma chamber and a chamber wall (which functions as an anode) to produce a continuous plasma through volume ionization of the background gas. A separately biased target is inserted into the plasma for ion implantation. The background gas pressure is typically 10.sup.5 - 10.sup.4 Torr. The plasma envelops all of the surface irregularities of the target object, which is pulse-biased to a high negative potential (on the order of 20-100 kV) with respect to the plasma by a high voltage modulator system that is independent of the plasma production system. The pulse-biasing of the immersed target reduces arcing, limits expansion of the plasma sheath, and achieves an omnidirectional and uniform implantation of plasma ions over the entire target surface. Major disadvantages, however, are that a separate plasma production system is required, and that sustaining the plasma on a continuous basis during the target pulsing results in a high ion surge current during the rise times of the voltage pulses. Furthermore, to achieve high ion doses in a short time period, a very high frequency operation of the target pulse modulator (on the order of 1 kHz) is required, which can lead to arcing. The ion implantation rate could also be increased by increasing the gas pressure above 10.sup.-4 Torr to raise the plasma density, but this can also result in arc formation.
In the Conrad patent the plasma sheath is initially close (a few Debye lengths) to the target surface. Upon application of the implantation voltage pulse, the plasma ions are removed from the plasma and the sheath expands. During the OFF time of the implantation voltage pulse, the plasma sheath moves back toward the surface of the part. In conventional practice, there is an upper limit of frequency (.about.1 kHz) of the implantation voltage pulses above which the plasma sheath is unable to move back in time, to conform to the target surface, before the next pulse is applied. Furthermore, with the plasma present before the implantation voltage pulse is applied, a very high electric field develops (&gt;50-100 kV/cm) across the plasma sheath once the implantation voltage is applied. A high ion current spike occurs which, together with the high electric field stress, leads to arcing at high repetition rate (&gt;1 kHz) and high gas pressure (&gt;10.sup.-4 Torr).
An alternate plasma production mechanism for plasma ion implantation is disclosed in patent application Ser. No. 07/595,123, filed Oct. 10, 1990 by Matossian and Goebel and assigned to Hughes Aircraft Company, the assignee of the present invention. In this application the plasma is formed in a localized plasma generator attached to the vacuum system, rather than by volume ionization of the background gas inside the ionization chamber. It has the advantage of improved uniformity in the implantation working space, improved selective plasma production from more than one species, the elimination of surface contamination from evaporation or sputtering of the filaments used in the U.S. Pat. No. 4,764,394 approach, and the control of the species mix for diatomic working gases. However, this system still utilizes separate discharge and cathode heater power supplies in addition to the high voltage modulator, requires multiple penetrations of the ion chamber, and uses a continuous plasma production that results in a high surge current during the rise time of the target pulse.
Other ion systems are known that operate at a much lower voltage regime than that discussed so far, generally at less than 1 kV, for coating a target rather than implanting ions into it. In one such system, described in U.S. Pat. No. 5,015,493 to Gruen, a glow discharge is used to produce ions in the gas. The ions are then attracted to the target surface with the same voltage signal that was used to create the glow discharge. Negative voltage pulses of between 0.1 and 1 kV are applied to the target, with pulse durations of about 10-100 microseconds at a repetition frequency of about 1-10 kHz. Pulses are used rather than a continuous signal to avoid depletion of plasma near the target surface, arcing, and overheating of the target surface during deposition. A specific gas pressure is not given, although the patent states that it "may be less than 100 Pa" (0.73 Torr).
While the Gruen patent is effective for coating a work piece by an ionized vapor, the principles upon which it operates are not applicable to the much higher voltages required for ion implantation. This is because a glow discharge such as that used in the Gruen patent depends upon direct collisions between secondary electrons emitted from the target, and the surrounding gas atoms to produce the ions necessary to sustain the plasma. At the much higher voltages employed for ion implantation, typically on the order of 50 kV or greater, the secondary electrons have a much longer mean free path and a correspondingly lower probability of making an ionizing collision with a gas atom. Thus, the ion density from secondary electron collisions will be much lower at the higher voltage levels. This is illustrated in H. Tawara and T. Kato, "Total and Partial Ionization Cross Sections of Atoms and Ions by Electron Implant", Atomic Data and Nuclear Data Table, Academic Press, Vol. 36, No. 2, March 1987, pages 167-353, in which the ionization cross-sections of the noble gases at energies greater than 20 keV are shown to be on the order of thirty times less than at 100-200 eV; at such low ionization cross sections it would be very difficult to produce and sustain a dense plasma. The drop in ionization would be even greater at 100 keV, at which much ion implantation work is performed.
A pulsed glow discharge has also been used for ion nitriding, in which nitrides are formed at the target surface from an ionized nitrogen gas. This technique is described, for example, in Kwon et al., "A Comparative Study Between Pulsed and D.C. Ion Nitriding Behavior in Specimens with Blind Holes", Proceedings of the International Conference on Ion Nitriding, 1987, pages 77-81. In this technique the gas pressure is even higher, on the order of 1-10 Torr, and again the voltage levels employed to establish a glow discharge (typically less than 1 kV) are much lower than those used for ion implantation.
Other pulsed ion systems are also known. In U.S. Pat. No. 3,732,158 to Przybszewski a 2-5 kV DC voltage source is initially connected to an object to establish a glow discharge used to sputter clean the object, followed by lowering the gas pressure to about 10.sup.-2 Torr, pulsing the voltage source and applying RF power to a film material which is sputtered onto the target. In U.S. Pat. No. 3,479,269 to Byrnes, Jr. et al. a sputter etching process is accomplished by applying a negative 1.5 kV pulse train to a target to both form a plasma and to attract ions to the target (see FIG. 5). However, as with the Gruen patent, the systems described in these two patents are not applicable to much higher voltage ion implant techniques.